68 MEMOIRS NATIONAL ACADEMY OF SCIENCES. [vol.xh. 



and distinctness. They vary also from partial to complete fusion so that the entire whorl is 

 of a nearly uniform deep purple color. Such purple shells are very rare ( = lurida Reeve) . These 

 color variations have not been the subject of special investigation, but they are mentioned 

 because of the possibility of their also showing Mendelian inheritance. 



2. ONTOGENY AND PHYLOGENY. 



As has been pointed out, Lang's experiments with inheritance in Helix are our main 

 guide in a knowledge of heredity in moUusks. He has shown that the same kinds of 

 variation which appear as individual may be inherited so that between fluctuating vari- 

 ations and marked variations or "mutations" there is only a distinction of degree; the 

 colonies or strains are found in nature, which by experiment have been shown to breed 

 true; that many characters show Mendelian inheritance in its typical form, and that in others 

 the sphtting takes place in the first generation of hybrids (Fj). Bandlessness is dominant over 

 the banded, red shell color over yellow, and this dominance has been observed to change with 

 age, as when yellow and red shelled individuals are crossed -with brown, the young were first 

 yellow and later became fully brown. On the other hand, dextrality is not inherited in Mende- 

 lian fashion. But as we have races of both dextral and sinistral mollusks, it is evident that 

 such traits are inherited and possess individuaUty. Lang further showed that the same general 

 forms of inheritance existed between races of species as between species. 



In the absence of experimental evidence of the manner of inheritance in lo, let us consider 

 the observations recorded on other animals and see what light these results throw upon our 

 subject. That spinosity is inherited is evident from the spinosity of the young shells from 

 the Nolichucky, and that dijBferent degrees of it may be inherited is indicated by the local races, 

 which vary in this character. The mixed character of the inheritance or expression, seen in the 

 Hoston shells from the vicinity of Rogersville (lots 87 and 88), suggests Mendelian inheritance 

 with a change of dominance with age. When yellow and brown shelled Helix were crossed, 

 yellow was first dominant and later brown. In the supposed crosses between smooth and 

 spinose lo, it looks as if the spinose trait was first dominant and later the smooth became domi- 

 nant. As a descriptive term this form of development was called "inverse." This change 

 of dominance or expression with age has been observed in many localities where relatively 

 smooth and spinose shells are found together. This change of expression, or change of domi- 

 nance with age, appears to be a hybrid characteristic and has been observed in many organisms. 

 It has been observed in hybrid oaks by MacDougal (1907, p. 48). Standfuss (1896, pp. 66-115) 

 has shown that in some hybrid moths, the larvse first resemble the maternal species and with 

 age approach the paternal. Davenport (1910, p. 131) has shown that when white and black 

 leghorn chickens are ci'ossed, the white is dominant, but it is so imperfect in the females that 

 they may be blue, and it is only in the later moults that they become nearly pure white. Guyer 

 (1909, p. 725) reports that in guinea-chicken hybrids the young at first resemble the guinea, 

 but with age become intermediate and still later, at the age of 5 years, even more nearly 

 approach the chicken parent (a male). Tower (1910, p. 298) found that in the cross produced by 

 two species of potato beetles the larvse were all of the female type, while the adult beetles which 

 developed from these larvse were intermediate between both parents. Giard (1903) reports 

 similar observations for moths and birds. Thomson (1908, p. 114) states that a boy or girl 

 may change with age from resembling one parent to the other; and Newman (1908) has shown 

 that in fish hybrids the dominance [or expression] of either parent may fluctuate and change 

 more or less with the age of the hybrid. Tower's (1910) experiments on hybrid beetles further 

 show that dominance may be changed by environmental influences. Some plants growing 

 under unfavorable conditions (ShuU, 1908, p. 446) do not show or express all of their inherited 

 characteristics, but do show more under favorable conditions. It is thus evident that the 

 change of dominance with age is frequent in hybrids and is also found in other organisms, and 

 must be considered in connection with the degree of expression of inherited characters. When 

 we consider that inverse development in lo is confined to locaHties where the smooth and spinose 

 shells are not only found together, but is best shown in those localities where all degrees of 



